It has been established from the incoherent inelastic neutron scattering spectra (IINS) that both the oxonium [H₃O]⁺ and dioxonium [H₅O₂]⁺ ions are present in the antimony hydrogen phosphate H₁Sb(PO₄)₂·2H₂O, written as H₁SbP₂·2H₂O. However, as the extent of hydration decreases as in H₁SbP₂·1.25H₂O, the proportion of the oxonium [H₃O]⁺ ion increases. MAS NMR indicates that the P—O^–H⁺ group remains undissociated in the anhydrous form. The title compound is an efficient but unselective ion exchanger which is able to extract mono- and poly-valent aquo-cations. In addition, polyvalent complex cations may be exchanged directly without the need for a catalytic or delamination process. A relatively high temperature (266 °C) is needed to dehydrate Co[Sb(PO₄)₂]₂·6H₂O in which the cobalt appears to be bound to four water molecules with two axial oxygens from the macroanionic layers.